In the field of wireless communication technology, a wide variety of products are used in offices and homes which implements, for example, wireless LAN communication according to the IEEE802.11 standards or wireless PAN communication according to the wireless USB standard. The above-described wireless communication technology uses a carrier communication method. Wired communication technology such as Asymmetric Digital Subscriber line (ADSL) also uses the carrier communication method. In the carrier communication method of wired or wireless communication technology, quadrature or orthogonal modulation/demodulation is widely used, especially for multi-bit modulation/demodulation.
For descriptive purposes, the following example describes a case of communication using the 802.11 wireless LAN standard, which utilizes a digital modulation method such as Orthogonal Frequency Division Multiplexing (OFDM).
At a transmitter side, a primary modulation wave having a plurality of subcarriers is generated based on a baseband signal. The baseband in-phase signal (“I signal”) and quadrature-phase signal (“Q signal”) are upconverted to a radio frequency (RF) band through a quadrature modulator. More specifically, the quadrature modulator mixes the I and Q signals (“I/Q signals”) respectively with the in-phase (0-degree) and quadrature-phase (90-degree out of phase) components of a local signal output from a local oscillator, and sums the I and Q signals to generate a composite RF signal. The RF signal is then output through an amplifier or antenna to an air interface as an electric wave.
At a receiver side, the RF signal that is received, for example, through the antenna, is input to a filter in which the high frequency components are removed. The RF signal is then mixed with the in-phase and quadrature-phase components of the local signal, which are output from the local oscillator and amplified by a low noise amplifier (LNA), and output as the baseband I/Q signals.
As an alternative to converting directly from the RF signal to the baseband signal, the RF signal may be converted to an Intermediate Frequency (IF) signal before being converted to the baseband signal. Even in such case, the modulator or demodulator is provided with the function to upconvert or downconvert from or to the baseband signal, and the function to apply quadrature modulation or demodulation.
While the quadrature modulation may be performed by a digital circuit, an analog circuit is often used for quadrature modulation especially when the frequency is made higher. For example, the RF band used for wireless communication is often processed by an analog circuit, while the baseband is processed by a digital circuit.
When the digital circuit is used for quadrature modulation, it is not likely to have errors, for example, due to changes in temperature or manufacturing process. However, when the analog circuit is used for quadrature modulation, the quadrature errors may occur due to various environmental factors including, for example, amplitude error, phase error, or DC offset. Not only do quadrature errors arise in the quadrature modulator, but quadrature errors may be caused due to the delay time in transmitting the signals as the I/Q signals pass the baseband filters. Since quadrature errors may cause an error in communication, the quadrature errors need to be corrected.
While the quadrature errors caused in the quadrature modulation analog circuit may be corrected either by an analog circuit or a digital circuit, the digital circuit has been widely used as it is capable of suppressing the overall system cost.
FIGS. 1 and 2 each illustrate a digital circuit for correcting the quadrature errors such as the amplitude error, the phase error, or the DC offset of the I/Q signals.
The transmitter system of FIG. 1 includes a baseband (BB) digital circuit 100, a correction circuit 101 including an I gain control 101a, a Q gain control 101b, and an IQ phase control 101c, and a digital analog converter (DAC) 102. The BB circuit 100 generates I and Q signals which differ in phase by 90 degrees. The I signal output by the BB circuit 100 is multiplied by a gain correction coefficient as it passes the I gain control 101a, and further multiplied by a phase correction coefficient as it passes the IQ phase control 101c. The I signal is then added with the Q signal, and further added with the DC component of the Q signal to produce the corrected Q signal. The Q signal output by the BB circuit 100 is multiplied by a gain correction coefficient as it passes the Q gain control 101b, and further multiplied by a phase correction coefficient as it passes the IQ phase control 101c. The Q signal is then added with the I signal, and further added with the DC component of the I signal to produce the corrected I signal. The corrected I/Q signals are converted from digital to analog at the DAC 102 to be transmitted into air as an electric wave.
The receiver system of FIG. 2 includes the baseband (BB) digital circuit 100, a correction circuit 103 including an I gain control 103a, a Q gain control 103b, and an IQ phase control 103c, and an analog digital circuit (ADC) 104. Based on the data signal received from the outside through the antenna, the I/Q signals are generated and input to the ADC 104 for conversion from analog to digital. The I signal is added with the DC component of the I signal, and multiplied by a gain correction coefficient as it passes the I gain control 103a. The I signal is then multiplied by a phase correction coefficient as it passes the IQ phase control 103c, and added with the Q signal to produce the corrected Q signal. The Q signal is added with the DC component of the Q signal, and multiplied by a gain correction coefficient as it passes the Q gain control 103b. The Q signal is then multiplied by a phase correction coefficient as it passes the IQ phase control 103c, and added with the I signal to produce the corrected I signal. The corrected I/Q signals are input to the BB circuit 100.
In order to correct quadrature errors, the value of the circuit, or the correction coefficients used for correction, may be determined as follows. In the transmitter system of FIG. 1, the power envelope components are extracted from the RF signal obtained by orthogonally transforming the I/Q signals output by the BB circuit 100 to generate a power envelope signal. The power envelope signal is fed back to the digital circuit to correct the IQ quadrature errors. Assuming that the quadrature error components are introduced into the local signal generated at the transmitter system of FIG. 1 due to various errors such as IQ gain error, IQ phase error, or IQ DC offset, the power envelope signal has a tone at a frequency f2 that is twice a frequency f1 of a single tone of the local signal. Assuming that the DC offset component is introduced into the local signal due to the DC offset error, the frequency f1 of the tone of the local signal increases. Assuming that the IQ gain or phase error component is introduced into the local signal due to the IQ gain or phase error, the frequency f2 of the tone of the envelope signal increases. Based on the detected values of the frequency f1 and the frequency f2, the value of the digital correction circuit may be determined so as to minimize the values of the frequencies f1 and f2.
In the receiver system of FIG. 2, the signal having a value equal to the local signal used for modulation at the transmitter side is input. The I/Q signals, which are obtained by orthogonally transforming the received RF signal, are input to the BB circuit 100. Based on a comparison between the power of the local signal and the power of the I/Q signals, the IQ gain error may be estimated. Further, the average of the product sum of the I and Q components may be obtained so as to adjust the value of the circuit to minimize the phase errors. Assuming that there are IQ quadrature errors, the value of the circuit may be determined so as to minimize the image data signal components in a frequency domain. The value used for DC offset correction at the receiver system of FIG. 2 may be obtained by averaging the I/Q signals that are input over a predetermined time period.
As an alternative to the above-described digital circuit for correcting the quadrature errors, the quadrature errors may be corrected in different ways, for example, as described in U.S. Pat. No. 7,035,341, U.S. Patent Application Publication No. 2008/0159442, or Japanese Patent Application Publication No. 2008-167057.
Japanese Patent Application Publication No. 2008-167057 describes an apparatus and a method of correcting phase and amplitude distortion using a Fast Fourier Transform (FFT) circuit, which is generally provided in the OFDM modulation/demodulation circuit. While it is relatively easy to calculate the power of the single tone having the same frequency with the frequency bin obtained from the relationship between the FFT and sampling frequency using the FFT circuit, the FFT circuit that is generally provided in the baseband modulation/demodulation circuit may not have sufficient compensation power to allow correction of quadrature errors. In order to reduce the manufacturing cost and electric power consumption, the FFT circuit provided in the modulation/demodulation circuit is usually designed so as to have the minimum computation power that is sufficient for modulation/demodulation, which may be determined based on quantized noise obtained for secondary modulation. On the other hand, the computation power required for calculating the power of the image signal caused due to the quadrature errors should be set greater than the computation power that is required for modulation/demodulation. In order to increase the computation accuracy, a correction circuit may need to be provided in addition to the FFT circuit that is already provided in the modulation/demodulation circuit. However, providing the additional circuit will cause the overall circuit size to increase, thus increasing the manufacturing cost and electric power consumption.